Fix Windows documentation build error handling 🤖🤖 - #15475

Fixes #15455

Summary

• propagate failures from the documentation generators and Sphinx on Windows
• keep incremental builds working when the target output directory already exists
• return a non-zero exit status for failed builds and missing Sphinx

Validation

• ran docs/make.cmd html twice with a compiled stub that returns success for the generator and exit code 7 for Sphinx
• verified both invocations returned exit code 1 and did not report a successful build

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